Most of current semiconductor devices, e.g. transistors, diodes, logic ICs or memory ICs are made from silicon (Si). Besides silicon, compound semiconductors, e.g. gallium arsenide (GaAS) and indium phosphor (InP) have been used for a limited number of applications, e.g. optical devices, ultra high speed ICs. Silicon, gallium arsenide and indium phosphor are excellent semiconductor materials.
However, these semiconductor materials cannot be used at high temperature. Silicon semiconductor devices do not work above 200.degree. C. Gallium arsenide semiconductor devices do not work above 300.degree. C. The incapability of the current semiconductor materials at high temperature is caused by their narrow band gaps. The band gaps of silicon and gallium arsenide are 1.1 eV and 1.5 eV respectively. Because of the narrowness of the band gap, these semiconductors enter into the intrinsic region above the temperatures, in which carrier concentrations increase. Excessively high carrier concentrations drastically lower the resistivity of the devices. Thus, the semiconductor devices which enter into the intrinsic region will be broken down soon.
Furthermore, the integration density of integrated circuits has been increasing year by year. Because of the increasing integration density, heat production per unit volume in devices increases also. The big heat production with insufficient heat diffusion often leads to misoperation of the devices.
To solve these difficulties, the Japanese Patent Laying Open No. 59-213126 and the Japanese Patent Laying Open No. 59-208821 proposed new semiconductor devices made from diamond with high heat resistance and with high heat diffusivity.
Diamond has many inherent advantages. First, diamond is chemically very stable. Second, diamond has a very wide band gap (5.5 eV). The band gap is so wide that the intrinsic region does not exist below 1400.degree. C., in which diamond is thermally stable. Here, the "intrinsic" region is the region of temperature in which the electron concentration is nearly equal to the hole concentration. In general, a semiconductor has three different types of current condition--n-type, p-type and intrinsic. The n-type semiconductor has electrons as majority carriers. The p-type semiconductor has holes as majority carriers. The intrinsic semiconductor has the same concentration of electrons and holes. The product (pn) of the electron concentration (n) and the hole concentration (p) is constant. According to the rising of temperature, the product (pn) increases. The product (pn) is simply written as EQU pn=(const).times.(kT).sup.3 exp (-Eg/kT)
where T is the absolute temperature, Eg is the band gap of the semiconductor and k is the Boltzmann constant. An n-type or a p-type semiconductor is made by doping an n-type dopant or a p-type dopant. However, in the case of semiconductors with narrow band gap Eg, a small rise in temperature increases the square root of (pn) rapidly. When the intrinsic carrier concentration h (h=(pn).sup.1/2) increases over the dopant concentration, the electron concentration becomes equal to the hole concentration regardless of the original n-type or p-type dopants. The difference of the concentration between the majority carrier and the minority carrier disappears. In this state, the semiconductor devices do not work. This state is expressed by "entering into the intrinsic region".
Third, diamond has very high heat diffusivity. The heat diffusivity of diamond is 20 W/cm K, which is ten times as much as that of silicon.
Forth, diamond is superior to silicon in the carrier mobilities. The electron mobility is 2000 cm.sup.2 /Vsec and the hole mobility is 2100 cm.sup.2 /Vsec at 300 K. The dielectric constant .epsilon. of diamond is 5.5. Diamond is gifted with high breakdown field E.sub.B =5.times.10.sup.5 V/cm. These properties of diamond heighten the speed of signals transmitting in diamond semiconductors. Higher breakdown field enables us to apply bigger input signals or controlling signals, which leads to the high speed transmission of signals.
Since diamond has the advantages; wide band gap, chemical and physical stability, high heat diffusivity, high carrier mobilities and high breakdown voltage, the semiconductor device having semiconductor diamond would have been a device which excels in the heat resistance and the environment resistance and works well at high temperature.
The diamond as an active part of a semiconductor device must be a single crystal. A single crystal diamond can be epitaxially grown as a film on a single crystal diamond substrate or on a single crystal silicon substrate from the material gas including hydrogen gas and hydrocarbon by the CVD (Chemical Vapor Deposition) methods. During the epitaxial growth of the diamond layers as films by the CVD methods, doping pertinent dopants, e.g. boron (B) or phosphor (P), we can obtain a p-type diamond or an n-type diamond with low resistivity.
However, the electrical property of the doped semiconductor diamond considerably changes according to the temperature of the environment. Unlike silicon, the doping levels of diamond are so deep that only a part of dopants supply an electron or a hole to the conduction band or to the valence band at room temperature. Here, the doping level means either the electron or hole states, or the energy levels of the electron or hole states of the dopants. In the latter case, the doping level of an electron is measured from the bottom of the conduction band and the doping level of a hole is measured from the top of the valence band. A "shallow doping level" means that the difference between the doping level of an electron and the bottom of the conduction band or the difference between the doping level of a hole and the top of the valence band is very small. In the case of silicon, usual dopants B, P, As or Sb make shallow doping level (about 0.01 eV) which is smaller than the thermal energy kT, where k is the Boltzmann constant and T is the absolute temperature. At room temperature the thermal energy is about 0.025 eV. If the dopant level Ed is smaller than kT, the carrier (electron or hole) of the dopant is fully excited to the conduction band or to the valence band. Thus, the carrier concentration is nearly equal to the dopant concentration in the case of silicon, because the dopants form shallow doping levels.
On the contrary, in the case of the dopant which forms a deep doping level (Ed&gt;kT), some of the dopant atoms release their electron or hole and become ions. The released electrons or holes act as free carriers. But other dopant atoms cannot release their electrons or holes. The number of the former active dopant is proportional to exp (-Ed/kT). Thus, if the doping level is deep (big Ed), the active dopant is very small. The carrier concentration is far smaller than the dopant concentration. Therefore, desirable dopants are such dopants that form shallow doping levels in the semiconductor.
Unfortunately in general, the doping level of semiconductor diamond is considerably deep. Because of the deep doping level, only a part of dopant atoms release their electrons or holes at room temperature. Other dopant atoms are still neutral atoms. To distinguish the two kinds of dopants, the former dopant atoms which are ionized are named active dopant atoms and the latter dopant atoms which are neutral are named passive dopant atoms. When the temperature is rising, the passive dopant atoms release their electrons or holes and convert to the active atoms. Thus the carrier concentration rises, and rapidly the resistivity decreases.
For example, boron-doped diamond shows the properties of a semiconductor even up to 500.degree. C. It has been confirmed that a transistor made from boron-doped diamond acts as a transistor at 500.degree. C. But the resistivity of the boron-doped diamond at 500.degree. C. is about one third of the resistivity at room temperature. Namely, the resistivity changes considerably according to the change of temperature. Therefore, if diamond semiconductor devices are used in the environment where the temperature changes up and down frequently, the electric property of the device is unstable.
Besides the instability for the temperature fluctuations, the diamond semiconductors have another problem that the carrier mobility decreases and the property of the interface of junctions deteriorates, when the doping amount is increased in order to accomplish larger current gain. When the dopant atoms are doped in the lattice of diamond, the dopant atoms induce lattice defects and fluctuations of the potential for electrons and holes. The lattice defects deteriorate the crystalline property of the interface of diamond. The fluctuation of the potential for carriers raises the possibility of scattering of carriers and decreases the carrier mobility.